Why 8k? Because it is future proof. If you shoot your project now in 2k then you may have to reshoot in 4k in a few years and then in 8k in a few years.
On the other hand, if output is in 2k, then 8k provides 4 times digital zoom that can be applied afterwards. Sharp images can be taken from a 8k clip and can be printed out to a meters wide poster.
Did I mention HDRx video capability?

------------------------------------------------------------------------------------------------------------------------------------------------------
*All prices are without VAT. $ 5% insurance is added. # The following rental day costs 10% less than previous day (applies up to 5 days).